1. Field of the Invention
The present invention relates to laser Doppler velocimeter apparatus for measuring velocity information, and more particularly to a laser Doppler velocimeter incorporating heterodyne detection for image sensing.
2. Description of Background Art
The present invention is related both Doppler Global Velocimetry and also to heterodyne detection.
Doppler Global Velocimetry (DGV) is a technique described in U.S. Pat. No. 4,919,536 issued Apr. 24, 1990 to Komine that views a laser-illuminated scene with a camera-pair system that derives two-dimensional instantaneous velocity information from images of the Doppler scattered light. The narrow-band laser is tuned to one side of an absorption line of a medium like molecular iodine. One camera of the pair views the scene through a filter cell containing the absorption medium and therefore records an image onto which velocity information has been impressed. A second reference camera image is needed to remove the original intensity dependent scene information by image division. The normalized image intensity provides a measure of the Doppler shift at each pixel. Alignment of the two cameras' sensors is therefore required within subpixel accuracy.
Heterodyne detection functions by coherently mixing a local oscillator (LO) wave with signal light at the sensor surface. The LO wavefront must match the signal wavefront closely enough so that there is only one interference fringe across the sensor area. The spectral content of the detected signal is the difference frequency between the LO and illumination laser sources plus the Doppler frequency shift of the scattered signal light. A prime advantage of the technique is that the detection sensitivity is quantum limited if the LO power dominates all sources of noise.
Velocity measuring systems incorporating lasers, and heterodyne laser sensors with local oscillators are known in the art. References include the following patents.
U.S. Pat. No. 4,822,164 issued Apr. 18, 1989 to Breen entitled OPTICAL INSPECTION DEVICE AND METHOD discloses a closed optical gauge that directs a laser beam toward an object whose radial velocity, instantaneous position, and/or shape are to be measured, and utilizes the Doppler frequency shift of the reflected beam to make measurements. A reference sample of the original direct beam has its frequency shifted by an acoustooptical modulator. A closed loop system is created by sensing the difference in frequency between the reflected beam and the shifted reference beam. The amount of frequency modulation is then controlled so that the frequency of the shifted reference beam continuously tracks that of the reflected beam, preferably with a fixed offset of frequency. The output can be integrated with respect to time to obtain position or shape information.
Absolute distance to the object is measured by transmitting a train of pulses of light toward the object and receiving reflected light pulses back. The round trip time of the light pulses is ascertained by measuring the phase displacement of the envelope of the reflected signals relative to the envelope of the transmitted light signals, with the aid of a phase locked loop. The oscillator of the phase locked loop is locked to the envelope of a reference sample of the transmitted signal, and can operate selectively at the fundamental frequency or a higher harmonic frequency of that envelope. A selected harmonic from the oscillator can be compared as to phase with the corresponding harmonic of the reflected light pulses. With calibration, the system can measure flight time, and therefore distance to the object, with high resolving power at the higher harmonic frequencies.
U.S. Pat. No. 5,110,207 issued May 5, 1992 to Harris entitled OPTICAL SYSTEM INCLUDING INTERFEROMETER FOR ENHANCING THE PERFORMANCE OF A PHOTOCONDUCTIVE NON-SHOT NOISE LIMITED DETECTOR describes an interferometer for transmitting a beam of electro-magnetic energy toward a target, portions of the transmitted beam reflected by the target being directed onto a detecting surface of a detector element with a local oscillator beam to provide a beat frequency signal representative of, for example, target range. The interferometer comprises a first lens for focusing the target-reflected return beam onto substantially a point on a detecting surface. The interferometer additionally comprises a second lens, for example, for focusing the local oscillator beam through a focal point onto the first lens. The first and second lenses are disposed to align such focal point of the second lens with a focal point of the first lens to thereby direct the local oscillator beam onto the detecting surface of the detector element with a substantially uniform intensity distribution thereon. With such arrangement, the detector element may be implemented as a photoconductive, rather than a photo-voltaic, detector with only a minimal net increase in the noise equivalent power (NEP) thereof, such as 5 dB, resulting in similar slight reduction in the signal-to-noise ratio of the interferometer. Since such photoconductive detector element may be cooled by thermoelectric stacks, the requirement for liquid nitrogen coolant is eliminated, thereby significantly reducing the size and weight of the interferometer system.
U.S. Pat. No. 5,123,730 issued Jun. 23, 1992 to Holmes et al. entitled APPARATUS FOR OPTICAL REMOTE WIND SENSING discloses apparatus that includes a light source for producing a coherent beam of light. A beam splitter is provided for splitting the beam of light into a first, transmitted beam segment and a second, local oscillator beam. A mechanism is provided for frequency shifting the frequency of one or both beam segments and for directing the first beam segment to a target. A remote target for scattering the first beam is provided. A mechanism is provided for combining the scattered first beam segment returning from the target and the second beam segment into a combined beam, and detecting the combined beam. A detector is operative to generate a signal indicative of the crosswind along the path of the directed first beam segment. A mechanism for determining the wind speed normal to the path from the generated signal is also provided.
U.S. Pat. No. 5,150,170 issued Sep. 22, 1992 to Morris entitled OPTICAL PHASE CONJUGATE VELOCIMETER AND TRACKER discloses an apparatus and method for determining velocity and range of a target within a field of view. A velocimeter and tracker includes a laser that produces modulated coherent light, a portion of which illuminates targets coming into a field of view of the velocimeter and tracker. Coherent light reflected from the target travels back along a detection path toward a phase conjugate mirror as a probe wave. A higher intensity portion of the coherent light produced by the laser is split into equal parts that are directed toward the phase conjugate mirror from opposite directions and interact with the probe wave to produce a phase conjugate light signal that travels back along the detection path and is also reflected from the target. Light reflected by the target experiences a Doppler phase shift as a function of target velocity. First, second, and higher order phase shifted signals reflected from the target are imaged on photo diodes, producing an electrical signal having components corresponding to the frequency differences of the various order light signals. As a function of the sums and differences of the frequency components of the electrical signal, the range and velocity of the target are determined.
U.S. Pat. No. 5,164,948 issued Nov. 17, 1992 to Nettleton et al. entitled STABILIZED LOCAL OSCILLATOR FREQUENCY FOR HETERODYNE LASER SENSORS discloses a stabilized local oscillator frequency for heterodyne laser sensors comprised of an acousto-optic crystal which provides a constant frequency offset f.sub.1, to a portion of laser transmitter power at frequency f.sub.0, for injection locking a separate local oscillator connected to a detector which simultaneously mixes f.sub.1 with the return signal f.sub.0 from a target. The detector outputs the difference frequency between f.sub.0 -f.sub.1 which is processed for the desired information. Proper local oscillator tuning is maintained through the use of a piezo-electric translator regulating the axial length of the local oscillator.
U.S. Pat. No. 5,187,538 issued Feb. 16, 1993 to Iwamoto et al. entitled LASER DOPPLER VELOCIMETER describes a laser Doppler velocimeter which has an optical system using a semiconductor laser as a light source for receiving a scattered light by reflection from natural particulate in fluid to be measured passing interference fringes formed in a region to be measured by a semiconductor optical detector, and a signal processor having a converter for converting a Doppler signal from the semiconductor optical detector to a predetermined waveform (e.g., a square wave) according to at least two different threshold values, and an arithmetic unit for comparing the processed signals from each other to judge that the scattered signal is from one natural particulate or not (i.e., the propriety of its measuring accuracy) and to calculate the velocity of the fluid to be measured according to the judged result and the process result of the converter from the period of the square wave when judging "a normal measurement".
U.S. Pat. No. 5,192,979 issued Mar. 9, 1993 to Grage et al. entitled METHOD AND APPARATUS FOR RECOGNIZING AND IDENTIFYING TARGETS teaches a method and apparatus that detects and identifies targets using a laser and operating in accordance with Doppler radar techniques. A laser sensor comprises a laser transmitter and an optical superheterodyne receiver and detects characteristic surface vibrations of targets and derives, from the laser echo signals, after frequency conversion and demodulation, low frequency oscillations having the frequencies of the vibration spectrum, from which, through comparison with patterns of known targets, provides detection and identification of the target.
U.S. Pat. No. 5,216,477 issued Jun. 1, 1993 to Korb entitled EDGE TECHNIQUE FOR MEASUREMENT OF LASER FREQUENCY SHIFTS INCLUDING THE DOPPLER SHIFT discloses a method for determining the frequency shift in a laser system by transmitting an outgoing laser beam, receiving an incoming laser beam having a frequency shift, acquiring a first signal by transmitting a portion of the incoming laser beam to an energy monitor detector and a second signal by transmitting a portion of the incoming laser beam through an "edge" filter to an edge detector, deriving a first normalized signal which is proportional to the transmission of the "edge" filter at the frequency of the incoming laser beam, deriving a second normalized signal which is proportional to the transmission of the "edge" filter at the frequency of the outgoing laser beam, and determining the frequency shift by processing the first and second normalized signals.
U.S. Pat. No. 5,229,830 issued Jul. 20, 1993 to Ishida et al. entitled DOPPLER VELOCIMETER describes an apparatus for detecting the speed information of an object that comprises a light source device, a frequency shifting device for shifting the frequency of a light flux from the light source device, a light-flux dividing device for dividing a light-flux whose frequency is divided by the frequency shifting device, one of the divided light fluxes divided by the light-flux whose frequency is divided by the frequency shifting device, one of the divided light fluxes divided by the light-flux dividing device being projected onto an object to be measured and the other light flux being projected in a direction other than that of the one of the light fluxes, a first detection device for detecting light from the object to be measured to which the one of the light fluxes is projected, the light detected by the first detection having undergone a Doppler shift proportional to the speed of the object to be measured, a second detection device for detecting the other light flux, and a speed information detection device for detecting the speed information of the object to be measured on the basis of the comparison between the detection results of the first and second detection devices.
U.S. Pat. No. 5,233,403 issued Aug. 3, 1993 to Mermelstein entitled HETERODYNE ARRAY FOR MEASUREMENT OF TARGET VELOCITY teaches a system for determining the angular velocity of a mobile target relative to a station. The system develops first and second reference beams and a beam to the target to enable the target to generate first and second signals. Optical devices, preferably fiberoptical devices, responsive to the first and second reference beams and to the first and second signals respectively received from the target at discrete first and second positions on the station, respectively produce first and second interference pattern signals. Each of said received signals has a Doppler shift as a function of the movement of the target. An electrical processing circuit then processes the interference pattern signals to develop a signal indicative of the target's angular velocity.